`int getlogin_r(char *buf, size_t len)` broken

Please use the following bug reporting template to help produce actionable and reproducible issues:

• A brief description
  Attempting to use the function to get the login id of the user running the executable returns garbled strings, with a different return value each time.
• Expected results

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

int main(){
        size_t len = 10;
        char buf[len];
        size_t ret = getlogin_r(buf, len);
        puts(buf);
        return 0;
}

should print out my username, with ret set to 0 to indicate success.

• Actual results (with terminal output if applicable)

The value of ret is 2, indicating an error.
Depending on the C compiler used, it outputs garbage strings.



If compiled with GCC or Clang, it prints out a single "unknown char" box.
If compiled with TCC, it prints out a string the size of one's username, composed of aforementioned boxes and random numbers, letters, and symbols.

• Your Windows build number

14393

• Steps / commands required to reproduce the error

gcc test.c -o test && ./test

• Strace of the failing command

Command doesn't fail in this sense.

• Required packages and commands to install

sudo apt-get install build-essential

Start by running the reported gcc test.c -o test && ./test reproduction and inspect the getlogin_r(char *buf, size_t len) entry point used by test.c. Confirm the behavior across the stated compilers, then make the call return 0 and populate buf with the current username without garbled output.